In recent years, there have been proposed and developed various electronically-controlled power steering systems each employing a reversible pump and a hydraulic power cylinder to provide steering assistance. One such hydraulic power steering system has been disclosed in Japanese Patent Provisional Publication No. 2002-145087 (hereinafter is referred to as “JP2002-145087”), assigned to the assignee of the present invention. In the hydraulic power steering system disclosed in JP2002-145087, downstream ends of a pair of hydraulic-pressure lines are respectively connected to left and right pressure chambers defined on both sides of a piston slidably accommodated in a hydraulic power cylinder. On the other hand, the upstream ends of the two hydraulic-pressure lines are connected to respective discharge ports of a reversible pump. The resulting pressure differential applied to the two sides of the piston, produces a steering assistance force. The magnitude and sense of the steering assistance force are determined based on the magnitude and sense of the steering torque applied to a steering wheel by the driver. Also provided is a communication passage or a bypass passage through which the two hydraulic-pressure lines, that is, the left and right pressure chambers, are intercommunicated with each other, when at least one of the reversible pump and the motor both incorporated in the power steering system is failed. A directional control valve (a shut-off valve), such as a two-position, spring-offset, two-way spool valve, is disposed in the communication passage, to block fluid communication between the two hydraulic-pressure lines via the communication passage during normal steering operation. On the contrary, in presence of a power steering system failure, such as a reversible pump failure or a motor failure, the directional control valve is shifted to its open position for fail-safe purposes, so as to permit full fluid-communication between the two hydraulic-pressure lines via the communication passage so that the power cylinder is held in the free state and does not produce an assisting force, and that a manual steering mode is ensured.
However, as a directional control valve, the power steering system disclosed in JP2002-145087 uses a single solenoid-actuated two-port two-position spool valve. There is a problem of oil leakage from a slight clearance space defined between the outer periphery of each land machined to slide in a very close-fitting bore of the spool valve body and the inner periphery of the bore, that is, the difficulty of ensuring a desired fluid-tight seal performance of the directional control valve, in other words, an undesirable drop of the steering assistance force from a desired value in the power steering mode. Additionally, there is a possibility of the spool sticking due to contamination or debris. With the spool stuck in the closed position, it is impossible to realize the manual steering mode in presence of the power steering system failure. From the viewpoint of reduced oil leakages and contamination and enhanced reliability of the hydraulic power steering system, the more enhanced fluid-tight seal performance and the more reliable operation of the directional control valve would be desirable.